Stainless ferritic chromium steels are used for many purposes as structural steels owing to their high resistance to corrosion, in particular by oxidising media, at high levels of strength and toughness and because they are cheaper than austenitic alloys. The resistance to corrosion depends on the chromium and carbon contents of the individual steel. While the resistance to corrosion improves with increasing chromium content the presence of carbon leads to the formation of chromium-rich carbides which precipitate at the grain boundaries and there lead to chromium depletion. This depletion in chromium is associated with impairment of the corrosion resistance in view of the aforementioned connection between the chromium content and corrosion resistance, and as a result the ferritic steels are more or less susceptible to intercrystalline corrosion, depending on the carbon content. To counteract this efforts are made to keep the carbon content in ferritic steels as small as possible, or at least to combine it stably with titanium. This, however, is associated with a considerable loss in toughness and resistance to corrosion and with the formation of titanium oxide, which impairs the surface quality, toughness and hot-workability.
Low carbon contents are, however, associated with the disadvantage that the transformability, which is dependent upon the carbon content, is lost, so that it is not possible to obtain a desirable balance of high strength and adequate ductility and toughness by mean of heat treatment. Therefore the room temperature strengths of non-transformable ferritic chromium steels only differ marginally.
To obtain higher strengths it is known from German patent 29 23 532 to first soft-anneal a stainless ferritic chromium steel after hot-rolling and then to cold work it with an 18 to 25% reduction. The annealing temperature is usually about 750.degree. to 850.degree. C., since higher annealing temperatures are associated with grain-coarsening, which results in loss of toughness and difficulties in cold working.
In addition the known process is both relatively costly in view of the need for soft-annealing and subsequent cold-rolling, and results in tensile strengths of only 750 to 800 N/mm.sup.2 as well as a considerable loss of ductility owing to the relatively large amount of coldworking.